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Topic: Dice probabilities (split from New Review)
Started by: Lxndr
Started on: 8/12/2003
Board: Adept Press


On 8/12/2003 at 2:18pm, Lxndr wrote:
Dice probabilities (split from New Review)

I read through the review, finally (took me a while) and, though it mostly looked like a good review, I noticed these little quirks, which are counter to what I have learned and believed about the game. I don't really have much to discuss, but I'd like clarification on these two points:

The Review wrote: So, if one player rolls 5, 3, 3, 2 while another rolls 4, 2, 2, 2, the first player has 3 victories.


It's only one victory, right? Not 3? Player 1 only rolled one die higher than a 4 (the highest of player 2).

The Review wrote: What kind of dice does Sorcerer use? That is entirely up to the players and GM, so long as all the dice are the same type. Just bear in mind that, the more sides your die of choice has, the harder it is for someone with low stats to defeat someone with high stats.


I was told in a recent thread (and I believe the book says so too, but I don't have the book here at work with me) that the smaller the dice size, the greater the skew towards a higher statistic winning. Now she's saying "the bigger the die, the less chance the lower-stat person has of beating the higher-stat person."

I guess what I'm asking is, in both situations, am I mistaken, or is the reviewer, or are we both?

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On 8/12/2003 at 3:19pm, Valamir wrote:
RE: Dice probabilities (split from New Review)

1) she was wrong. Its only 1 success.

2) Sorcerer mechanics have a fairly high chance of the lower pool winning. A single lucky 10 against an opponent who rolls all 9s and 8s is good enough for victory. Contrast this with Target Number and count successes dice pools where given the same TN the odds skew much more heavily towards a bigger pool.

In other words you have to be lucky in Sorcerer for a 3 die pool to beat a 7 die pool...but not as lucky as you'd have to be in Riddle of Steel.

Smaller die sizes increase the likely hood of ties. If the high dice are tied you go to the second and then the third etc. That means when ties are involved it isn't enough for the small pool to get a single lucky die...you have to get several lucky dice. The more likely ties are the more of an advantage the larger pool has, because the more likely the smaller pool is to run out of high dice...or in the extreme...run out of dice altogether.

So smaller die sizes make "upset" rolls less likely. Larger die sizes make "upset" rolls more likely. The difference even between d10 and d6 is pretty noticeable in this regard. Ties become common place with dice pools of 8-13 dice rolling d6s.

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On 8/12/2003 at 5:38pm, rafial wrote:
Sorcerer die mechanics redux

Valamir wrote: Sorcerer mechanics have a fairly high chance of the lower pool winning.
....
In other words you have to be lucky in Sorcerer for a 3 die pool to beat a 7 die pool...but not as lucky as you'd have to be in Riddle of Steel.


This is correct. 3:7 would have about a 25% chance of success in Sorcerer. I don't know what the TROS odds are off the top of my head (or if TN makes a difference), but I'm guess its is much smaller.

So smaller die sizes make "upset" rolls less likely. Larger die sizes make "upset" rolls more likely.


I'm not sure that is true. The only factor that controls the overall probability of success or failure in Sorcerer is the relative size of the die pools. The number of successes produced is influenced by the absolute size of the die pools (2:1 has the same odds as 4:2, but 4:2 may produce more successes) and also the size of the die used (smaller dice increase the odds of extreme results). The die size effect becomes most noticable when the number of dice in the largest pool exceeds the number of faces on the die. So it seems to me that smaller die sizes would make upset rolls *more* likely, since the smaller pool has the same odds of winning regardless of die size, but has slightly higher odds of pulling more successes if they *do* get a victory.

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On 8/12/2003 at 7:05pm, Tim Alexander wrote:
RE: Dice probabilities (split from New Review)

Hey Rafial,

I'm not sure that is true. The only factor that controls the overall probability of success or failure in Sorcerer is the relative size of the die pools.


Does that really work out? I'm a little rusty on my probability, and I haven't run all the numbers, but it seems to me that Ralph's assessment seems more on target for this. If we're rolling d4s and I have one die, and you have four, I've got a 25% shot at getting a four, and the probabilities for you are pretty much a win. If we're in the same situation with d20s, I've got a 1/20th shot, and you've got 1/5th. My overall odds are worse, but my odds of getting a 20 when you haven't, have increased, right?

Like I say I haven't run all the numbers and the devil is always in the details when it comes to heavy permutation situations like these, but am I missing something more obvious than that?

-Tim

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On 8/12/2003 at 7:17pm, Valamir wrote:
RE: Dice probabilities (split from New Review)

Tied dice are removed from the pool.

In a 3 vs 7 situation, the larger pool is 2.33 times bigger than the smaller
If the high die ties the pools become 2 vs 6 and the larger pool is 3 times bigger.
If the second high die ties the pools become 1 vs 5 and the larger pool is 5 times bigger.

Thus each time the high die ties, the odds of winning between the pools that are left increases for the larger pool.

Since smaller die sizes increases the likelyhood of ties, they increase the number of times during the game the above scenario plays out.

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On 8/12/2003 at 7:25pm, Mike Holmes wrote:
RE: Dice probabilities (split from New Review)

Ralph's original statment is true, smaller dice favor the winner. This is well documented. That said, the effect is not really very pronounced, amounting to a shift of, at most, a couple of percent. And the effect gets less and less as the dice increase in sides. d20 v d100 is less different than d6 v d10 (ties just become even more infinitesmally unimportant). You probably ought to stay away from flipping Coins, though even that wouldn't be too big a deal. To get any real distortion in the curve, you'd have to roll dice that had some result that occured more often than .5, meaning doing something silly like rolling a d6 and counting 1-5 = 0, and 6 = 1.

Basically, given Sorcerer's priorities, it's a complete non-issue. The system works just as well, overall, with any dice.

Mike

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On 8/12/2003 at 7:34pm, Valamir wrote:
RE: Dice probabilities (split from New Review)

I'd have to see the numbers crunched to buy that Mike. My admittedly rough stab at it indicates that the effect...while not overwhelming, is significant enough to be noticeable to the "naked eye".

While I agree fully that the system works just as well with any dice, I am at this point convinced that players who play several sessions with d10s and then switch to d6s will experience a noticeable reduction in the number of times the smaller pool wins.

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On 8/12/2003 at 7:45pm, Ron Edwards wrote:
RE: Dice probabilities (split from New Review)

I'm all confused now. Are we talking about how often the one pool wins over the other, or the degrees of difference by which it wins?

Best,
Ron

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On 8/12/2003 at 9:22pm, rafial wrote:
sorcerer dice

In my case, I was talking about how often one pool wins over another. My results are based on a python program that I let run for several days (100,000 rolls if I remember correctly). If there is an effect of die size on how often one pools wins over another, it is less than the "noise" in my results, >1% over the range of d6-d20, with pool sizes ranging from 1-10 dice. The only variable I saw affect the overall odds of which pool won was the ratio of the pools.

I totally grant that the absolute size of the pools and the die type used affect the degree of difference by which one pool will win over another.

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On 8/12/2003 at 10:39pm, Tim Alexander wrote:
RE: Dice probabilities (split from New Review)

Brilliant! This is why I'm never good with these problems. I inevitably attempt to do it formulaically and can't remember the method in anything but the simplest of cases, for whatever reason I never think to do it experimentally. Does your script just dump pure winning percentages, or does it give an indication of the margin of victory averages as well?

-Tim

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On 8/12/2003 at 10:52pm, rafial wrote:
experimental odds

talex wrote: Does your script just dump pure winning percentages, or does it give an indication of the margin of victory averages as well?


It does give margin of victory, in fact I even have nifty graphs made with GnuPlot, but the data I have right now on margin of victory is not correct for this discussion, since it uses the Donjon varient of adding back matches to the number of success after victory is determined. This inflates the average number of successes on victory, but does not alter the overall win/lose percentages.

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On 8/12/2003 at 11:14pm, Tim Alexander wrote:
RE: Dice probabilities (split from New Review)

Very slick. In the interest of not entirely highjacking the review thread (though we're a bit far gone at this point,) I'll drop you a private message. I'm a perl monkey, not a python guy, but I'd like to have a gander at it to see about modding it for Sorcerer. I think it'd be useful to get some probability info on the margin of winning at various die sizes.

-Tim

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On 8/12/2003 at 11:20pm, rafial wrote:
Mia culpa

talex wrote: Very slick. In the interest of not entirely highjacking the review thread (though we're a bit far gone at this point,) I'll drop you a private message. I'm a perl monkey, not a python guy, but I'd like to have a gander at it to see about modding it for Sorcerer.


It's a one line change. Don't add back the ties.

However, I'm glad you brought it up, because it caused me to go and pull up the data files that are sitting on my disk. I hadn't looked at them at a long time, and in reinspecting them I see my original contention that the overall die size does not affect overall win/lose percentages may not be correct. In fact, it looks like a smaller die actually increases the chances slightly for a smaller pool. I'll look at the data in more detail tonight.

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On 8/13/2003 at 4:57pm, Mike Holmes wrote:
RE: Dice probabilities (split from New Review)

I've done the same experiment a couple of times using spreadsheets. And repeatedly I've found the same results.

In any case, what Rafial and I both agree on in terms of results is that the differences in outcome are so small as to be unimportant. A player isn't going to notice less than a 5% difference in frequency of success. And the differences are always less than that. We can quibble about the fine details if we like, but that's the only important conclusion to be drawn.

Even if it did make a larger difference, would it matter then? Which way makes more "sense" for Sorcerer? Disparity in pools meaning more likely success, or less likely success? I can't think of any reason why it's pertinent.

Mike

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On 8/13/2003 at 5:16pm, rafial wrote:
die pool calculations

Mike you are correct. I'm baffled by how badly I misinterpreted my data when I first generated it six months ago.

I regenerated my data sets last night using the sorcerer method of discarding ties, and built the graphs into a web page for everyone's amusement.

Pool ratio is still definitely strongest overall factor in determining odds of success, but its now clear to me that die size does have an effect, which grows in step with absolute pool size. In the ranges I ran (1-12 dice), the difference in outcomes between d20 and d6 looks to be around 5%, with smaller die sizes favoring the larger pool, as originally stated on this thread. I stand corrected.

Also, larger absolute pools at the same ratio favor the larger pool by a slight margin.

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On 8/13/2003 at 6:07pm, Mike Holmes wrote:
RE: Dice probabilities (split from New Review)

Those charts ought to convince anyone (assuming they can read a chart) that the differences are unimportant.

BTW, Ralph, in Donjon, the differences are way more pronounced because of the effect of ties, so maybe that's what you're thinking of. That's a completely different game going from d20 down to d10 (or, heaven forfend, d6).

Mike

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On 8/13/2003 at 7:40pm, Valamir wrote:
RE: Dice probabilities (split from New Review)

Ok, I hate to belabor this issue, but the graphs aren't really helping. They're just graphing numbers without any indication of the accuracy of the numbers generated. There are certain things about how the graphs are displayed that red flag them in my mind (note: red flag =/ they're wrong. red flag = more information please).

First the graph of 5:5 and 6:6 are exactly what I'd expect. All lines line up perfectly. The tie effect comes from changing the ratio of the pool size, so with equal pool size, there is no effect. However 3:3 and 2:2 ARE showing an effect. Since I can't think of a good mathematical reason for why the graph of 3:3 would look different from the graph of 5:5 both of which show the expected 50%, Red Flag is raised. Something is going on there I don't understand.

Second, I don't know how the number generation accounted for ties. I see a note where graphs were altered to make the data continuous, but this is erroneous. The graph cannot and should not be continuous. You have either 1 or more victories for the large pool (positive numbers) or 1 or more victories for the small pool (negative numbers). There should be a complete gap at 0 because ties do not stand. I'm not sure why this adjustment was made, but it confuses the heck out of me and raises the question as to whether the tied results are being accounted for correctly in whatever system is rolling the dice.


I went and prepared a grunt work spreadsheet where I simply listed every single possible combination of a 3 die d4 pool in the columns and ever single possible combination of a 2 die d4 pool in the rows, and then a second sheet using the same pools with d6s. I chose the small number of dice simply to keep the number of cells manageable. I suppose one could do a 8d10 vs 4d10 this way but it would be enormous.

The results were exactly what I expected them to be.

When the die size was dropped from d6 to d4 the number of times ties occured increased from 24.77% to 36.72%.

When ties occured on a d6, the smaller pool managed to win with subsequent dice 24.14% of the time.

When ties occured on a d4, the smaller pool managed to win with subsequent dice only 20.75% of the time

More ties occuring with a higher ratio of those ties going to the bigger pool is exactly what I've been saying.


As for how pronounced the effect is overall at these pool sizes it made a difference of 3.78%. Thats just going from d6 to d4.

The chance of rolling a double on 2d6 is 16.67%
The chance of rolling a double on 2d4 is 25%
That's 1.5x more likely dropping from d6 to d4

The chance of rolling a double on 2d10 is 10%
That's 1.67x more likely than rolling a double on 2d6
So I'd expect the difference to be even more pronounced going from d10 to d6s


As to whether a difference of 3-5% is significant enough to worry about. Well, that I guess resides in the eye of the beholder. Some people take their couple of % on D% very seriously, some do not.

I'f one were playing d20 and one found a sword that was +1 anytime the roll was even and +0 anytime the roll was odd (essentially +1/2) they'd prefer it over a straight sword...even though the statistical difference is only 2.5%

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On 8/13/2003 at 8:12pm, Mike Holmes wrote:
RE: Dice probabilities (split from New Review)

I'f one were playing d20 and one found a sword that was +1 anytime the roll was even and +0 anytime the roll was odd (essentially +1/2) they'd prefer it over a straight sword...even though the statistical difference is only 2.5%


Sure, because they can percieve that. The GM will tell them so.

But a player has to roll 20 times to even get a sample that will have the anomaly in it for a 5% difference. And then he has to go through that 20 roll cycle multiple times before there's a chance that they can see the trend that the odds are different. And this all assumes someone looking for it, or who has a reason to pay attention, and someone who has played with the other method. Sorcerer doesn't really give you a reason to pay attention to the dice. D&D is quite Gamist, so, of course, every little advantage makes a difference. In Sorcerer, you don't care what the results of the roll are particularly; they're only a "spingboard for cretivity" (hence the reason that the "unrealistic" underdog effect is cool).

So what does it matter if it's slightly different, one way or another?

Mike

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On 8/13/2003 at 9:09pm, rafial wrote:
odder and odder

Valamir wrote: They're just graphing numbers without any indication of the accuracy of the numbers generated.


Fair cop. Since the method being used is random sampling, the accurary is dependent on getting the number of samples high enough. We can get some idea of the amount of "noise" by looking at expected versus actual results in the simple cases. If you looke at 1:1, you see that the with 50:50 expected, we get 49.8:50.2 in one case and 49.9:50.1 in the others. So it looks like we've got around 0.2% error in the results.


First the graph of 5:5 and 6:6 are exactly what I'd expect. All lines line up perfectly. The tie effect comes from changing the ratio of the pool size, so with equal pool size, there is no effect. However 3:3 and 2:2 ARE showing an effect.


The effect being shown is that with smaller die sizes, there is a slightly lower probability of extreme differences. This trend actually carries into the higher numbers, it just becomes harder and harder to see as the tails get longer. Look at the raw data sets.


Second, I don't know how the number generation accounted for ties.


Do you mean tied dice, or tied pools? The function that scores a roll is doing exactly what you'd do by hand. tied high dice are set aside, and then the pool is re-examined. I keep a count of the number of tied dice to add back in for the donjon odds, but in the run that generated those graphs, ties were simply discarded per sorcerer.

If the one pool runs out of dice before a victory is scored, an additional die roll is added to each pool, and the pools are rescored. Correct me if this is not the proper sorcerer way to break ties.


I see a note where graphs were altered to make the data continuous, but this is erroneous. The graph cannot and should not be continuous. You have either 1 or more victories for the large pool (positive numbers) or 1 or more victories for the small pool (negative numbers). There should be a complete gap at 0 because ties do not stand.


There is a gap at 0. ties do not stand. "shifting up" the failure results was done simply as a workaround for gnuplot. The labels on the axis of the graph are then "shifted down" to cancel out the original shift. This has nothing to do with the original results generated, it is simply done for convenience of plotting. The only reason I mentioned this was if you look at the raw data, the value of "-1" on the graph will be found as 0 in the data set, and "-2" on the graph will be found as -1 in the data set and so on.


More ties occuring with a higher ratio of those ties going to the bigger pool is exactly what I've been saying.


I believe that is born out. My original arguments were based on my own faulty analysis of my data.

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On 8/15/2003 at 6:50pm, Ron Edwards wrote:
RE: Dice probabilities (split from New Review)

For reference's sake, all of the above posts were split from the New Sorcerer review thread.

Best,
Ron

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On 8/20/2003 at 8:31pm, djarb wrote:
Somewhat different results

I'm a new poster here, so howdy all :)

I've written and run a script similar to the ones described earlier in this thread, but the results are quite different. I'm seeing a marked advantage for the lower score when rolled with larger dice: for example, a pool of 3 versus a pool of 6 wins 18.8% of the time with d4, but 31.5% of the time with d20

I checked some of the simpler dice combinations (e.g 1 vs 2 for d2 and d20) against the theoretical probability, with results within a couple of percentage points of what the experiment turned up. I have reasonable confidence in these numbers.

You can find the results of the script here:
http://www.highenergymagic.org/sorcdice.txt

and the script itself here:
http://www.highenergymagic.org/sorcdice.py

If the effect of die face count is as strong as this indicates, perhaps some information about how they affect play should go into the FAQ and/or later editions of the game.

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On 8/20/2003 at 8:47pm, Lxndr wrote:
RE: Dice probabilities (split from New Review)

The book DOES say that the # of sides on the die makes a difference, and that the larger dice favor the underdog. I'm not sure if any more statistics are really needed (though they're nice to see).

Djarb: Did you just roll a # of times, randomly? And was it true-random (like with hexbits) or was it pseudo-random? Or did you take every single possible roll in every case, and compare them that way? To do it "right" (so to speak) I think you'd have to do the latter.

Or, I'm sure someone with more probability math knowledge than myself can figure out some sort of equation.

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On 8/20/2003 at 9:23pm, djarb wrote:
RE: Dice probabilities (split from New Review)

Lxndr wrote: The book DOES say that the # of sides on the die makes a difference, and that the larger dice favor the underdog. I'm not sure if any more statistics are really needed (though they're nice to see).



Ah, my bad.

Lxndr wrote:
Djarb: Did you just roll a # of times, randomly? And was it true-random (like with hexbits) or was it pseudo-random? Or did you take every single possible roll in every case, and compare them that way? To do it "right" (so to speak) I think you'd have to do the latter.

Or, I'm sure someone with more probability math knowledge than myself can figure out some sort of equation.


It was pseudo-random, seeded from the time, with a pseudo-random period of 2**19937-1. In case you don't know, that might as well be true-random for this purpose. If you're not convinced, I could use numbers from random.org

For each combination of die faces with dice pool sizes between 1 vs 1 and 9 vs 9 inclusive I rolled 10,000 times, recording who won and how many victories the winner got. I handled ties according to the rules and the errata, with ties that exhaust the smaller pool counted as 0-victory wins for the larger pool.

As to running an exhaustive search of the results: there are 167,960 ways to combine nine d20s, so for example 9 vs 9 d20 would produce 28,210,561,600 different rolls, and storing the results of that one combination would take up more than seven times the addressable memory on a 32bit computer, even if you only stored who won. Running the calculations that way would be a severe pain in the ass :)

A large random sample should be good enough, all things considered.

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